This application is based on and claims priority from Korean Patent Application No. 10-2003-0034808 filed on May 30, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of Invention
The present invention relates to a method for wireless local area network (LAN) communication in an ad hoc network, and more particularly, to a method for wireless LAN communication by which a station that has lost a channel reservation competition for a main channel can transmit data via another channel.
2. Description of the Related Art
In general, a wireless LAN refers to a wireless LAN based on IEEE 802.11 standards. IEEE 802.11 defines standards for a wireless LAN operating in the 2.4˜2.5 GHz ISM (Industrial, Scientific, Medical) band. The ISM band is a frequency band prescribed for the utilization of equipment in industrial, scientific and medical applications. The ISM band can be freely used without permission if the transmitting power is less than a predetermined level.
An IEEE 802.11 network is basically configured with a Basic Service Set (BSS) composed of several stations communicating with one another. The BSS includes an independent BSS in which stations perform direct communication therebetween without using, an access point (AP), and an infrastructure BSS in which an AP is used in all communication processes.
The independent BSS mainly consists of several stations constructed with a specific purpose and for a specific period of time. For example, the independent BSS includes a network established upon holding a meeting in a conference room. When the meeting begins, respective attendants will establish an independent BSS to share their data. Then, when the meeting ends, the attendants will break up the independent BSS. Because of the short life, small scale and specific purpose of such an independent BSS, it is often called an ad hoc BSS or ad hoc network.
FIG. 1 shows a wireless LAN in an ad hoc environment, which is configured with five stations.
First and third stations intend to transmit their data to second and fourth stations, respectively. The first and third stations that intend to send data transmit the data competitively via a channel reservation competition. As a result of the competition, only the station that wins in the competition is allowed to transmit the data.
FIG. 2 shows a conventional data transmission process of a wireless LAN in the ad hoc network of FIG. 1.
In FIG. 2, a beacon is a frame responsible for the notification of a presence of a network and maintenance of the network. The beacon is periodically transmitted so that a mobile station causes parameters to correspond thereto for participation in the network and finds and recognizes the network. In an infrastructure network, an AP performs the beacon transmission. In an ad hoc network, each station performs the beacon transmission and only a beacon of a station that has won the competition is transmitted. If a station that has received a beacon has data to be sent (i.e., MAC Protocol Data Unit: MPDU), the station generates an announcement traffic indication message (ATIM), and transmits the generated ATIM to notify the presence of the buffered data.
Referring to FIG. 2, it shows a case where the first station has won a competition with the third station. In particular, FIG. 2 shows that the first and third stations have their data to be sent to the second and fourth stations, respectively, and intend to generate and transmit their ATIMs, but the first station has sent an ATIM first. The ATIM of the first station has been transmitted to the second to fifth stations, but the third station cannot transmit its ATIM to the fourth station. When an ATIM window, i.e. time period during which an ATIM can be transmitted, terminates, the third station that lost in competition and the fourth and fifth stations with no data to be sent enter a doze mode in order to save power. When it is time for the third to fifth stations in the doze mode send subsequent beacons, they return to an active mode in order to send or receive beacons in competition with the first and second stations. In active mode, the aforementioned process is repeated again.
The data transmission of the first and second stations is performed after the ATIM window terminates. To avoid a collision that may be caused by any hidden stations, the first station intending to send data transmits a request-to-send (RTS) while the second station receiving the data transmits a clear-to-send (CTS) so as to reserve a transmission medium or to assure continuous data transmission. When the RTS and CTS are thus exchanged, the first station transmits the data and the second station acknowledges the received data. The data transmission may be performed on the basis of several fragments, and a station that has received each fragment sends an acknowledgement (ACK) in response thereto. A short interframe space (SIFS) is provided between associated frames such as RTSs and CTSs, data, and ACKs for reception of data.
FIG. 3 shows a conventional data transmission process of a wireless LAN in the ad hoc network of FIG. 1.
In FIG. 3, a case is shown where the third station has successfully sent an ATIM to the fourth station and the first station also has successfully sent an ATIM to the second station. In this case, the first to fourth stations are all in active mode. If the ATIM window terminates, the first and third stations competitively send their data. At this time, if the first station first transmits the RTS, the first station takes priority of transmission of data to the second station. Due to the presence of a network allocation vector (NAV) in the RTS packet, the third station cannot transmit data for a period of time during which the first station is sending and receiving the data. After the first station completes data transmission to the second station, the third station can transmit the data only if time to receive a beacon still remains. Meanwhile, although the RTS and CTS are packets used in overcoming problems with any hidden nodes, they may be omitted. FIG. 3 shows that upon transmission of data to the fourth station, the third station transmits data while omitting transmission of the RTS and CTS.
As described above, there is a need for a method and apparatus for allowing a station to transmit data even though the station loses a competition in an ad hoc network. That is, in the case of FIG. 2, the third station cannot transmit data to the fourth station. In the case of FIG. 3, the third station cannot transmit data during a predetermined period of time in the network allocation vector. Further, even though it is possible to implement a method and apparatus for allowing a station to transmit data even when the station loses a competition, there is a problem in that if the implementation method greatly deviates from existing IEEE 802.11 standards, a station meeting the standards cannot perform wireless communication with such a newly implemented apparatus. Therefore, there is a need for an improved method and apparatus for allowing a station to perform wireless communication with standard stations while meeting IEEE 802.11 standards.